LIGHT AVAILABILITY AND GROWTH OF WILDCELERY (VALLISNERIA AMERICANA) IN UPPER MISSISSIPPI RIVER BACKWATERS

Large beds of Vallisneria americana declined in the backwaters of the Upper Mississippi River after a drought that occurred between 1987 and 1989. One hypothesis for this decline is that low light availability may have decreased net photosynthesis to the extent that overwintering tubers were not formed. Following the decline, light availability rem ained low. To determine what light levels would be necessary for the r e-establishment of Vallisneria in the Upper Mississippi River, the lon g-term growth of plants in a backwater lake and in an experimental pon d was measured while the surface and subsurface light were monitored c ontinuously. Plants grown from tubers transplanted to 0.5, 1.0 and 1.5 m depth in the lake grew and produced tubers only at 0.5 m depth (9% of surface light). At 1.0 m, light availability was less than 1% of th e surface light. Plants grown from tubers in experimental ponds with f our shade treatments (2, 5, 9 and 25% of surface light) for the same g rowing period produced replacement-weight tubers in 9% light. For a lo nger growing season, plants also produced replacement-weight tubers in treatments with at least 5% of surface light. An average light-extinc tion coefficient of 4.64 m(-1) was calculated for the backwater lake b ased on continuous data collected during 94 days during the growing se ason from eight widely separated sites. Using equations based on the a verage extinction coefficient for the lake and average leaf lengths of plants grown in experimental ponds, we predict that in years with com parable turbidity, plants grown from locally collected tubers will gro w and produce replacement tubers only at depths of 0.8 m or less.